1. Field of the Invention
The present invention relates to perpendicular recording and read head assembly with in situ stand alone stabilizer for a magnetic medium underlayer and, more particularly, to such a stabilizer which stabilizes the magnetic medium underlayer below the read head without impacting the operation of the read head.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm and an actuator arm. When the disk is not rotating the actuator arm parks the suspension arm on a ramp. When the disk rotates and the slider is positioned by the actuator arm above the disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm positions the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A write head is typically rated by its areal density which is a product of its linear bit density and its track width density. The linear bit density is the number of bits which can be written per linear inch along the track of the rotating magnetic disk and the track width density is the number of tracks that can be written per inch along a radius of the rotating magnetic disk. The linear bit density is quantified as bits per inch (BPI) and the track width density is quantified as tracks per inch (TPI). The linear bit density depends upon the length of the bit along the track and the track width density is dependent upon the width of the second pole tip at the ABS. Efforts over the years to increase the areal density have resulted in computer storage capacities increasing from kilobytes to megabytes to gigabytes.
The magnetic moment of each pole piece of a write head is parallel to the ABS and to the major planes of the layers of the write head. When the write current is applied to a coil of the write head the magnetic moment rotates toward or away from the ABS, depending upon whether the write signal is positive or negative. When the magnetic moment is rotated from the parallel position, magnetic flux fringing between the pole pieces writes a positive or a negative bit in the track of the rotating magnetic disk. As the write current frequency is increased, the linear bit density is also increased. An increase in the linear bit density is desirable in order to increase the aforementioned areal density which increase results in increased storage capacity.
There are two types of magnetic write heads. One type is a longitudinal recording write head and the other type is a perpendicular recording write head. In the longitudinal recording write head the flux induced into first and second pole pieces by the write coil fringes across a write gap layer, between the pole pieces, and into the circular track of the rotating magnetic disk. This causes an orientation of the magnetization in the circular disk to be parallel to the plane of the disk which is referred to as longitudinal recording. The volume of the magnetization in the disk is referred to as a bit cell and the magnetizations in various bit cells are antiparallel so as to record information in digital form. The bit cell has a width representing track width, a length representing linear density and a depth which provides the volume necessary to provide sufficient magnetization to be read by a sensor of the read head. In longitudinal recording magnetic disks this depth is somewhat shallow. The length of the bit cell along the circular track of the disk is determined by the write flux frequency, write field gradiant, linear velocity of the track and the demagnetization field from the media. In longitudinal recording, the bit cell is lying along the circular track direction. The written transition (flux reversal) is in the same direction. However, the demagnetization field from the magnetic charge at the transition is in the opposite direction to the magnetization in the recorded bit. When the bit cell length becomes shorter and shorter, the demagnetization field will increase. Eventually, when the field strength is nearing Hc of the media, it will demagnetize the written transition by itself. It is this demagnetization effect that limits the minimum bit cell length achievable by the longitudinal recording scheme. In order to reduce the demagnetization field, the recording media layer thickness has to be reduced if the media remanent saturation magnetization remains the same, which results in smaller bit cell volume and weaker signal level during readback process.
In perpendicular recording, since the magnetization of the recorded bit cell is vertical to the surface, the demagnetization field generated by the neighboring bits is in the same direction as the magnetization direction of the current bit, which helps to stabilize the recorded transition. Therefore, one can write very narrow bit cell length along the circular direction without suffering from the demagnetization effect in the longitudinal recording case. As a result, perpendicular recording is expected to be able to achieve much higher recording bit density compared to longitudinal recording. Also, for the same reason, thicker media can be used to increase the signal level during readback, which reduces the amplitude requirement from the read sensor.
Since most perpendicular write heads use a single pole for writing, a high permeable soft underlayer is needed to provide a flux return path to increase the write field gradient and to improve the sharpness of the written transitions. After the recording layer of the rotating disk has been recorded by the write head the disk passes under the read head where the read head reads the recorded signal which is in a portion of the recorded layer directly below the read head. Unfortunately, a soft underlayer portion directly below the recorded layer portion typically introduces noise into the reading process which seriously degrades the error rate performance of the head assembly. The noise is due to the fact that the underlayer portion is in a multiple magnetic domain state and the domain walls move with the slightest provocation. Domain walls and domain wall movement induce magnetic flux changes in the read sensor during disk rotation. This introduces unwanted magnetic signals into the read head which causes degradation of signal to noise ratio (SNR) to recording channels.
A constant bias field has been proposed for maintaining the underlayer in a single domain state. In one proposal a bias field is introduced into the soft underlayer at a remote location from the read head during rotation of the magnetic disk. Unfortunately, by the time the underlayer portion reaches the read head the underlayer portion may have returned to a multiple domain state. Another approach is to employ components of the read head for stabilizing the soft underlayer. This approach will pose reliability issues such as sensor non-linearity, sensor stability, amplitude thermal decay and electron migration.